1. Field of the Invention
The present invention is related to a spin coating apparatus such as a spin coater for processing a round substrate such as a semiconductor wafer while supplying predetermined coating liquid on a top surface of the round substrate and rotating the round substrate, and is also related to a spin coating method.
2. Description of the Background Art
As shown in FIG. 13, in a conventional substrate processing apparatus, through a nozzle 2, resist liquid 3 (i.e., photoresist) is supplied as coating liquid onto a central portion of a semiconductor wafer (i.e., round substrate) 1 which is horizontally supported, while rotating the wafer 1 at a low speed of about 900 rpm as shown in FIG. 14. Due to weak centrifugal force which is created by the slow rotation, the resist liquid 3 which stays at the central portion is spread entirely over the wafer 1. At this stage, the resist liquid 3 is spread as two portions, i.e., an approximately round portion 6 which spreads almost uniformly in all directions (hereinafter "core portion") and a portion 7 which outwardly flows in the shape of whiskers (hereinafter "river portion") because of the centrifugal force which becomes locally stronger than surface tension of the core portion, as shown in FIGS. 15 to 17. In FIGS. 15 to 17, the symbol Q expresses a discharged flow quantity per unit time (per second) of the resist liquid 3, and the symbol .omega. expresses a rotation angular speed of the wafer during discharging.
Once the core portion 6 of the resist liquid 3 is spread to a certain range, the wafer 1 is rotated at a high speed to further spread the resist liquid 3 using the centrifugal force so that levelling is performed, that is, a film thickness of the resist liquid has a uniform in-plane distribution (i.e., a distribution of the film thickness on the substrate) as shown in FIG. 18. In FIGS. 13, 14 and 18, denoted at 4 is a supporting pedestal which fixes the central portion of the wafer 1 by suction force, denoted at 4a is a rotation shaft which is fixedly attached to the supporting pedestal 4 and driven to rotate by a motor not shown, and denoted at 5 is a drain chamber which is open at an upper portion.
In the conventional technique, conditions such as the discharged flow quantity, an discharging flow time and the rotation angular speed which are necessary to spread the resist liquid 3 all over the top surface of the wafer 1 are set by experiences, rather than based on a logical ground. Hence, in the conventional technique, when the wafer 1 is rotated at a slow speed (about 900 rpm) during discharging of the resist liquid 3, the resist liquid 3 of the core portion 6 spreads slower than the resist liquid 3 of the river portion 7, and therefore, a ratio of consumption of the resist between the core portion 6 and the river portion 7 is about 3:7. This means that the quantity of the resist which flows out without used is 2.3 times as large as the quantity of the resist which coats the top surface of the wafer 1. Thus, since 70% of the resist liquid 3 which is discharged through the nozzle 2 is not used, a large quantity of the resist is consumed, which in turn increases a cost. For instance, when a wafer having a diameter of eight inches is rotated at 900 rpm, assuming that the discharged flow quantity Q per unit time (per second) of the resist liquid 3 is 1 ml/sec, the discharging time t needs be about three to five seconds and a total discharged flow quantity is three to five mililiters.
Further, as the surface area of the core portion 6 is enlarged gradually at a slow rotation speed as in the conventional technique, a solvent evaporates during the spreading, whereby the spreading does not progress smoothly and therefore a processing time becomes even longer.